As the digital technology progresses recently, people convert a radiographic image into a digital image signal (the value of a pixel contained in the digital image signal will be referred to as a pixel value), execute image processing for the digital image signal, and display the image on a display device such as a CRT or output the image to a film.
When a radiographic image is sensed, generally, tone conversion processing is executed by using a linear LUT or an LUT with an S-shaped tone conversion curve representing the characteristic of a film to make the converted digital image signal suitable for display or film output.
In X-ray images of human bodies, the dynamic range of the subject changes depending on the individual difference between subjects or radiography conditions. Hence, to optimize the minimum and maximum values of the dynamic range of the subject in advance, the dynamic range of the subject is changed (compressed or expanded) (e.g., reference 1).
According to this method, processing is performed to make the dynamic range of the subject fall within the given dynamic range of the output medium. For this reason, the performance of the output medium can be maximized, and an output image easy to diagnose can be obtained.
In some cases, dynamic range compression processing is executed to make the dynamic range fit in that of the output medium while maintaining the contrast of a fine structure. An example of this method is described in, e.g., reference 2.
In this method, let SD be the pixel value of an image which has undergone processing, Sorg be the pixel value of the original image, and Sus be the average pixel value (pixel value of the smoothed image) obtained by calculating the moving average of the original image on the basis of a mask size of M×M pixels. The processing is executed bySD=Sorg+f(Sus)  (1)Sus=ΣSorg/M2  (2)using a monotone decreasing function f(x). Equation (1) can be rewritten to
                                                                        S                D                            =                            ⁢                                                (                                                            S                      org                                        -                    Sus                                    )                                +                                  (                                                            f                      ⁡                                              (                        Sus                        )                                                              +                    Sus                                    )                                                                                                        =                            ⁢                                                (                                                            S                      org                                        -                    Sus                                    )                                +                                  f                  ⁢                                                                          ⁢                  1                  ⁢                                      (                    Sus                    )                                                                                                          (        3        )            
Since Sorg−Sus corresponds to the high-frequency component of the image, this processing can also be regarded as dynamic range change processing limited to a low-frequency component. Hence, the fine structure corresponding to the high-frequency component can be maintained.
[Reference 1] “Computer Imaging Processing Approach” edited by Hideyuki Tamura, (Japan), Soken-Shuppan, pp. 95-97
[Reference 2] Anan et al, “Journal of Japan Radiological Society”, August 1989, Vol. 45, No. 8, p. 1030
In the method described in reference 1, however, when the dynamic range of the image is compressed excessively, the contrast may be too low.
Conversely, when the dynamic range is expanded excessively, the contrast may be too high. The compression or expansion processing is executed uniformly for the entire image. When viewed from the local region basis, an unnecessarily wide dynamic range may be assigned to a small information amount, or a narrow dynamic range may be assigned even when the information amount is large.
In the method described in reference 2, the dynamic range is changed for only a low-frequency component. Hence, even when the dynamic range is compressed excessively, no shortage of contrast occurs.
However, since the low- and high-frequency components are unbalanced, artifacts are generated by overshoot or undershoot. This is particularly conspicuous near an edge where the gradient of pixel value is large. That is, artifacts are generated when a low frequency is assigned to a narrow dynamic range in spite of a large information amount as an edge.